Lithium rechargeable batteries are classified according to the kind of electrolyte used. Thus, lithium rechargeable batteries are generally classified into liquid electrolyte lithium ion batteries and polymer electrolyte lithium ion batteries. Lithium ion polymer batteries using solid polymer electrolytes are advantageous since there is no danger of electrolyte solution leakage. Furthermore, lithium ion polymer batteries can be manufactured in an ultra-thin battery shape. As a result, there have been increased efforts to commercialize lithium ion batteries that use solid polymer electrolytes. In addition, lithium ion polymer batteries are typically lighter weight, exhibit lower vapor pressures and smaller discharge rates as compared to lithium ion batteries that use liquid electrolyte. Due to such characteristics, lithium ion polymer batteries are safer than their liquid electrolyte battery counterparts and can be easily manufactured into angular batteries or large-sized batteries.
Polymer electrolytes used in lithium ion polymer batteries generally include pure solid polymer electrolytes, gel-type polymer electrolytes, hybrid polymer electrolytes and the like. The pure solid polymer electrolyte produces a thin film by a solvent evaporation coating, with specific examples thereof including graft polyether electrolytes, polysiloxane electrolytes, and the like.
Gel type polymer solid electrolytes that produce a polymer host structure and a stable cell by the addition of an electrolytic solution typically exhibit higher ionic conductivities as compared to pure solid polymer electrolytes and thus yield better cell performance. In order to compensate for the weakness in mechanical properties of the gel-type polymer electrolytes, crosslinking or thermosetting materials are generally further added during their preparation. The ionic conductivity of the polymer solid electrolyte is based on the mobility of ion species in an electrolytic solution.
Examples of gel-type polymer solid electrolytes include electrolytes prepared by forming a mixture of ethylene glycol and dimethacrylate and then irradiating the mixture with ultraviolet (UV) radiation. While such an electrolyte exhibits excellent flexibility, it is liable to be hardened by heat after UV irradiation, which makes further processing impossible. In the case of fabricating a battery using a polymer electrolyte, the interface resistance between an electrode and electrolyte increases, thereby making it extremely difficult to use such an electrolyte in practice. Another example of a gel-type polymer solid electrolyte includes a crosslinked polyethyleneoxide electrolyte which is prepared by crosslinking polyethylene oxide to reduce crystallinity. As a result, the ionic conductivity of the electrolyte can be improved to a maximum of 10−5 S/cm, which is still unsatisfactory to be used as a room-temperature type lithium rechargeable battery. Another example of gel-type polymer electrolyte includes polyacrylonitrile-based electrolytes which are prepared by dissolving polyacrylonitrile in an electrolytic solution and making a gel with the temperature of the resultant structure further reduced. (See, Abraham et al, J. Electrochem. Soc., 137, 1657 (1990); and Passerini et al, J. Electrochem. Soc., 141 80 (1994), the entire content of each publication being expressly incorporated hereinto by reference). The thus obtained electrolyte, while exhibiting good ionic conductivity of 10−3 S/cm, has poor mechanical strength and does not exhibit uniform electrical impregnation characteristics.
Hybrid polymer electrolytes have been prepared by injecting an electrolytic solution into a porous polymer matrix having fine pores of less than sub-micron dimension. (See, U.S. Pat. No. 5,296,318, the entire content of which is incorporated hereinto by reference.) The pores of the polymer matrix serve the role as a pathway for the liquid electrolyte and which are thus filled with the electrolyte to give a good ionic conductivity reaching 3×10−3 S/cm. However, production of such a hybrid polymer electrolyte system requires a plasticizer extraction step, which requires a substantial amount of time (e.g., about 1 hour). Furthermore, a substantially large amount of organic solvent, such as ether or methanol, is needed to prepare the polymer matrix thereby necessitating a refining facility for recycling the used organic solvent.
As described above, in the case of using the gel-type polymer electrolyte and the hybrid polymer electrolyte, the polymer matrix must be impregnated with a relatively large amount of electrolytic solution to obtain a good ionic conductivity characteristic. However, existing polymer electrolytes still are unsatisfactory in terms of their ionic conductivities and battery assembly processes.
To solve the above problems, it is one object of the present invention to provide a composition for forming gel polymer electrolytes having improved ionic conductivity. Another object of the present invention is to provide a simplified rechargeable battery manufacturing process employing a polymer electrolyte using the composition for gel forming polymer electrolytes.
To achieve the objects of the present invention, there is provided a composition for gel forming polymer electrolytes having a gelling agent and liquid electrolyte. Heating a composition comprised of the gelling agent and liquid electrolyte gives an ionic structure on the crosslinked polymer chain which results in a high ionic conductive gel polymer. The gelling agent most preferably includes at least one material selected from polymers, copolymers, oligomers or monomers containing primary, secondary or tertiary amines as a nitrogen source. In addition, at least one material can be selected from polymers, copolymers, oligomers or monomers containing organic halides that that are capable of reacting with a nitrogen group at an elevated temperature. The gelling agents are dissolved into organic liquid electrolytes containing between about 0.5 M to about 2 M of ionic salts. The solution is then heated to an elevated temperature (e.g., between about 30 to about 130° C.) to prepare the gel polymer electrolyte. Surprisingly, the ionic conductivities achieved by the gel polymer electrolyte formed according to the present invention is up to 1×10−2 S/cm, which is a conductivity value similar to that of a liquid electrolyte.
To achieve the second object of the present invention, there is provided a lithium secondary battery having a cathode, an anode and a polymer electrolyte interposed between the cathode and the anode. A polyolefin (e.g., polypropylene or polyethylene) microporous separator sheet is used to separate the electrodes to prevent electrical shorting. The non-activated cell is placed into a suitable case (preferably a case formed of a thermoplastics material which is capable of being heat-sealed and which is inert to the cell contents). The gel electrolyte precursor liquid is then poured into the case. The case with the gel electrolyte precursor liquid therein is sealed and then subsequently heated to an elevated temperature and a time sufficient (e.g., about 65° C. for between about 1 to 7 days) to cause the gel precursor to gel (polymerize) in situ within the case thereby forming a rechargeable lithium ion polymer battery cell.
These and other aspects and advantages will become more apparent after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.